Building a web application in PHP and trying to decide on a database. MySQL seems to be the default choice for PHP but PostgreSQL supposedly handles complex queries better and has more features.

For a community/forum type site with maybe 10,000 users eventually, which would you recommend?

MySQL is the practical choice for PHP. Better documentation, more PHP tutorials use it, easier to set up, and more hosts support it. Unless you need advanced SQL features, go MySQL.

PostgreSQL is more standards-compliant and handles concurrent writes better. But for a forum with 10k users, MySQL 4.x will be absolutely fine.